A prior art method of preventing a wheel loader from slipping is disclosed in the Japanese Laid-Open Patent Application No. SHO 64-4013. According to this prior art method of preventing the loader from slipping, a slip controlling device conducts processing of information on the basis of an input signal from a detector for detecting the load on an implement, and outputs a command signal for controlling the implement and a command signal for controlling the force of traction upon generation of slip of the vehicle so that the output signals permit the implement to effect compensating operation so as to reduce the loading thereon.
Besides the above-mentioned prior art method, in general, methods of controlling the output of the engine or controlling the brake when the wheel loader is slipping could be envisaged.
Out of the above-mentioned prior art methods, according to the former method, since the load on the implement is automatically reduced when the vehicle is slipping, that is, no-load condition is reached, so that a situation against the intention of the operator will occur if this method is effected.
Further, according to the latter method in case wheel loader slips during excavating operation, even if the operator detects the occurrence of slip and takes procedures to control it, it takes about 0.5 to 1.0 second until the slip is restrained. Therefore, in case of the method of controlling the output of the engine, the response is slow, and in case of the method of controlling the brake, the braking action tends to be delayed, so that both the methods have been ineffective.